Concrete is valued for its strength, durability, and versatility, but it is not completely resistant to cracking. Even concrete used in a newly completed building, parking lot, roadway, or foundation may develop cracks shortly after placement.
Some cracks are minor and primarily cosmetic. Others may indicate problems with the concrete mixture, installation, curing process, soil conditions, or structural design. Understanding why concrete cracks can help project owners and contractors determine whether further evaluation is needed.
Plastic Shrinkage
Plastic shrinkage cracks can develop while concrete is still fresh. They occur when moisture evaporates from the surface faster than it can be replaced by water rising through the concrete.
Hot temperatures, low humidity, direct sunlight, and strong winds can increase the risk of plastic shrinkage. These cracks are often shallow and may appear as short, irregular lines across the concrete’s surface.
Proper planning and placement practices can help reduce moisture loss during this critical stage.
Drying Shrinkage
Concrete contains water that is necessary for mixing and placement. As the concrete hardens and excess moisture leaves the material, the concrete naturally shrinks.
When movement is restricted by the subgrade, reinforcement, walls, or other structural components, tensile stress can develop. Because concrete has relatively low tensile strength, cracking may occur as the material contracts.
Properly placed control joints help manage drying shrinkage by encouraging cracks to develop in planned locations.
Improper Curing
Curing allows concrete to retain sufficient moisture and temperature as it gains strength. If concrete dries too quickly, it may not develop its intended durability and strength.
Inadequate curing can contribute to surface cracking, dusting, reduced wear resistance, and other performance problems. The appropriate curing method and duration depend on the concrete mixture, environmental conditions, and type of construction.
Excess Water in the Concrete Mixture
Adding too much water can make concrete easier to place and finish, but it can also reduce its quality. As excess water evaporates, the concrete may shrink more than expected.
A high water-to-cement ratio can also decrease strength and increase porosity. Contractors should follow the approved mixture design and avoid adding unnecessary water at the jobsite.
Poor Subgrade Preparation
Concrete depends on the soil and aggregate beneath it for stable support. If the subgrade is soft, uneven, poorly compacted, or contains unsuitable material, it may settle after construction.
When sections of a slab lose support, the concrete may bend and crack under its own weight or under traffic and equipment loads. Proper soil evaluation, grading, moisture conditioning, and compaction testing can help identify and reduce these risks.
Settlement Beneath the Concrete
Even when the initial subgrade preparation appears adequate, later soil movement can contribute to cracking. Settlement may result from poorly compacted fill, changes in soil moisture, erosion, leaking utilities, or the decomposition of buried organic material.
Cracks related to settlement may be accompanied by uneven surfaces, sunken sections, or changes in elevation between different portions of the slab.
Thermal Expansion and Contraction
Concrete expands as temperatures rise and contracts as temperatures fall. If the concrete does not have enough room to move, internal stresses can develop.
Expansion joints, isolation joints, and other design features may be needed to accommodate movement. The number, placement, and spacing of joints should reflect the structure’s design and expected environmental conditions.
Incorrect Joint Placement
Control joints are intended to manage where shrinkage cracks occur. If joints are spaced too far apart, installed too late, or cut to an insufficient depth, cracks may form elsewhere.
Joint layout is especially important around corners, columns, openings, and changes in slab dimensions. A thoughtful jointing plan can reduce random cracking and improve the finished appearance.
Premature Loading
Concrete gains strength over time. Allowing vehicles, construction equipment, stored materials, or structural loads onto the concrete too soon may create stresses it is not yet strong enough to support.
Construction schedules should account for the concrete’s required strength before loading. Field-cured cylinders, maturity testing, or other testing methods may help determine whether concrete has developed sufficient strength for the next phase of work.
Reinforcement and Design Issues
Reinforcing steel, welded wire reinforcement, and fibers can help control cracking, but they must be properly selected and positioned. Incorrect reinforcement placement may limit its effectiveness.
Cracking can also result when a concrete element is not designed for the loads, span, thickness, or site conditions it will experience. Structural cracks may require evaluation by a qualified engineer.
Weather Conditions During Placement
Extreme temperatures can make proper concrete placement and curing more difficult. Hot weather may accelerate evaporation and setting, while cold weather can slow strength development and potentially expose fresh concrete to freezing.
Rain during placement can also affect surface quality, particularly if water is worked into the concrete during finishing. Contractors should monitor weather conditions and follow appropriate hot- or cold-weather concreting procedures.
When Should Concrete Cracks Be Evaluated?
Not every crack indicates a serious structural problem. However, further investigation may be appropriate when cracks:
- Continue to widen or lengthen
- Show vertical displacement
- Allow water to enter the structure
- Appear shortly after loading
- Occur alongside settlement or movement
- Develop in structural elements
- Exceed the project’s specifications or tolerances
Documenting the crack’s width, length, location, and changes over time can provide useful information during an evaluation.
Construction Materials Testing Can Help Protect Concrete Quality
Many causes of concrete cracking can be reduced through proper site preparation, mixture control, placement, curing, and quality assurance testing. Concrete testing can verify properties such as slump, air content, temperature, and compressive strength. Soil and aggregate testing can also help confirm that the supporting materials meet project requirements.
Quality Testing & Engineering provides construction materials testing and engineering services to support projects throughout the St. Louis metropolitan area and Southern Illinois. Contact QTE to learn how proper testing and inspection can help protect the quality and long-term performance of your concrete construction.